I've said for a long time that to cover the basics a person needs a center fire rifle, center fire pistol, shotgun and .22. That makes sense to most folks as it should; I stole the idea from Massad Ayoob who is a smart dude. For me those weapons would be an AR, a Glock 19, a Remington 870 and a Ruger 10/22. This is not to say a guy who chooses an FN-FAL, a Glock 21, a Mossberg 590 and a tube fed Marlin is at all wrong. Just doing things a bit differently.
If something happened and these guns were all I had that would be fine. Of course that is for ME. For a couple I would *like* both people to have a pistol and ideally a rifle of their own. These should be compatible when possible/ practical. The shotgun and .22 can be 'pool' weapons unless you need 2 at the same time for hunting or whatnot. A reasonable person could stop at 4 guns and be just fine. Some like redundancy by owning two or more of the same weapons platform and that is great if you have the money and inclination to do so. However moving past these 4 basic guns there are a few really useful things to have.
Depending on what the first rifle is a military pattern semi auto or a surplus/ bolt action hunting type rifle getting the other type really gives a lot of options. This is probably the biggest benefit of the semi auto .308's. You can really do both pretty well with any of the common 'battle rifles'. Or you could pick up a bolt gun to round things out and only have one cartridge. Regardless our first niche gun will be the scoped rifle (or semi auto if you had a scoped in the basic 4).
The next one that comes to mind is a really small pistol. Yes I coined the title myself. Genius I know. We could debate the exact top end of this size but I don't feel like putting that much thought into it. To me the universal snubby .38, Walther PPK and Baby Glock's are a little bit too big to really fit here. We are talking guns like the Kel Tech P3AT, S&W Bodyguard .380, Ruger LC380, baby Beretta's, NAA tiny revolvers, etc all.
Calibers range from .380 down to the good old .22 Long Rifle. Personally I do not see a huge point in arguing ballistics here. They all suck to varying degrees. Seriously it is a race to the bottom like a Toyota Prius owner arguing with a Smart Car owner and a Hyundai Accent owner about who has the best performance.
Strait Forward in a Crooked World makes a good case for the little .22 that I touched on awhile back. .22 ammo is uuber common and generally considered more benign, and thus easy to acquire, than other pistol calibers. Something to think about anyway. The new breed of little single stack 9mm's such as the S&W shield, Ruger LC9 as well as the older established Khar and Kel Tech's make a good case for skipping the .380 entirely.
After a lot of consideration these guns have a viable, if narrow role. Dressed in anything more than a Speedo these guns can be concealed. There is a lot to say for this ability. Carrying a small or really small pistol as a backup is something people do for various reasons. For cops and other various folks who tend to be at higher than average risk this is probably smart. For Joe Everyday CCW it probably isn't necessary but then again a little pistol in a pocket holster isn't a huge sacrifice.
Some folks carry these guns as a primary weapon. I am not a huge fan of this. As these guns (to varying degrees) are difficult to shoot well AND carry small amounts of weak cartridges it doesn't seem like the best idea to me. Certainly better than harsh words but if better options are realistically possible I would suggest them.
Not sure if I would go really small pistol before or after the scoped rifle (or semi auto) first. It probably depends on your situation. A dude living in the hinterboonies should probably get the other rifle while a suburbanite would be well advised to pick up a really small pistol.
Do you have a really small pistol? What role does it fill?